Canopy element influences on subgrid-scale energy within a large-eddy simulation

Enhancement of velocity gradients in the proximity of the individual elements of a plant canopy is responsible for the rapid transfer of kinetic energy from the mean flow and from canopy-scale turbulence to small-scale turbulence. Since wake-scale turbulence is likely to dissipate quickly, this process can act to short-circuit the normal inertial cascade route to dissipation. A number of higher-order closure models of canopy flow have accounted for this energy transfer process. Here, we examine subgrid-scale (SGS) models within the context of large-eddy simulation (LES). Unresolved turbulence enters through the action of SGS viscosity on resolved-scale velocity shear (inertial cascade), and also via the action of individual canopy elements in creating wake turbulence. SGS energy is normally characterized by the grid spacing, while element dimensions characterize wake-scale motions. It is also necessary to consider the possible importance of the transfer of kinetic energy from SGS energy entering via inertial cascade to wake scales. The specific impact of canopy elements on unresolved turbulence is expected to be very dependent on the relative magnitude of these two scales. This paper examines these questions within the framework of the LES of atmospheric surface layer flow including a canopy such as a forest.